1. Field of the Invention
The present invention relates to a solid-state image sensing apparatus.
2. Description of the Related Art
A solid-state image sensing apparatus is formed as, for example, a CMOS image sensor or CCD image sensor, and includes a pixel portion in which pixels are arranged in an array form and analog/digital conversion units which convert analog signals output from the pixel portion into digital signals. The analog/digital conversion units include comparison units which compare analog signals with comparison reference voltages and output the determination results and counter units each of which counts a clock signal to measure the time required from the instant comparison starts to the instant a determination result changes. It is possible to use a plurality of buffers to supply clock signals with uncollapsed waveforms to the counters of the counter units. An increase in the number of pixels leads to an increase in the number of counter units, and an increase in the length of a clock wiring for the propagation of a clock signal, which accompanies an increase in parasitic capacitance component/parasitic resistance component. This requires more buffers.
The use of a large number of buffers can cause a perturbation in the duty ratio (High period/signal cycle) of a clock signal due to manufacturing variation, in particular. Japanese Patent Laid-Open No. 2009-89066 discloses a circuit which counts each clock signal at its two edges. According to this literature, this circuit suppresses a perturbation in the duty ratio of counting operation.
An increase in the number of buffers with an increase in the number of pixels and manufacturing variation accompanying miniaturizing can cause a problem in terms of a perturbation in the duty ratio of a clock signal. According to patent literature 1, the disclosed technique suppresses a perturbation in the duty ratio of counting operation. However, this literature does not disclose any technique of suppressing a perturbation in the duty ratio of a clock signal itself.